Demonstrators protest against Syria's President Bashar al-Assad after Friday prayers in Yabroud, near Idlib. Photograph: Reuters

At least 90 people, including many children, have been killed after Syrian forces shelled and attacked the town of Houla in Homs province, according to anti-government activists.

The death toll reported on Friday was one of the highest in one area of the country since an internationally brokered ceasefire came into effect last month.

The violence began when security forces opened fire on an opposition protest in Houla, activists said. Anti-government forces retaliated and the army began shelling the area.

A local activist giving his name as Abu Yazan, reached via Skype, said 12 people died in shelling and 106 were killed when pro-regime thugs known as shabiha stormed the area.

"They killed entire families, from parents on down to children, but they focused on the children," he said.

Amateur videos posted online showed many children among the dozens of dead laid out in different rooms and covered with sheets and blankets. One video showed 14 dead children lined up on a floor, shoulder to shoulder.

A spokesman for the United Nations' envoy to Syria told the Associated Press in an email on Saturday that international monitors were travelling to Houla "as we speak" to investigate.

The UK-based Syrian Observatory for Human Rights issued an unusually harsh statement in the wake of the deaths, accusing Arab nations and the international community of being "partners" in the killing "because of their silence about the massacres that the Syrian regime has committed".

The surge in violence came as Ban Ki-moon, the UN secretary general, told the UN security council there had been some progress in reducing violence, but the overall situation remained very serious.

"There is a continuing crisis on the ground, characterised by regular violence, deteriorating humanitarian conditions, human rights violations and continued political confrontation," he reported to the security council in a letter.

In his report to the security council, Ban wrote that the size and sophistication of some recent bomb attacks in Syria suggest that "established terrorist groups" may have been behind them and urged groups and countries not to supply weapons to either side in Syria.

Ban said the UN observers noted that "significant parts of some cities appear to be under the de facto control of opposition elements".

"There is an overall atmosphere of tension, mistrust and fear," Ban said. "The overall level of violence in the country remains quite high."

In other areas of Syria, activists said that a further 33 people were killed. The Syrian government news agency said that 17 people had been killed by anti-government gunmen. None of the figures could be independently verified.